Dynamically Long-Term Imaging of Cellular RNA by Fluorescent Carbon Dots with Surface Isoquinoline Moieties and Amines

Carbon Nanomaterial Fluorescent Probes and Their Biological Applications

Fluorescent carbon nanomaterials have broadly useful chemical and photophysical attributes that are conducive to applications in biology. In this review, we focus on materials whose photophysics allow for the use of these materials in biomedical and environmental applications, with emphasis on imaging, biosensing, and cargo delivery. The review focuses primarily on graphitic carbon nanomaterials including graphene and its derivatives, carbon nanotubes, as well as carbon dots and carbon nanohoops. Recent advances in and future prospects of these fields are discussed at depth, and where appropriate, references to reviews pertaining to older literature are provided.

Wavelength-Tuneable Fluorescent Carbon Dots for Nucleic Acid Imaging

Nucleic acid is one of the most important substances in organisms, and its dynamic changes are closely related to physiological processes. Nucleic acid labeling is conducive to providing important information for the early diagnosis and treatment of pathophysiological processes. Here, we utilized the transfer mechanism between carbon sources and CDs to synthesize wavelength-adjustable N-CDs for the nucleic acid image. Along with the increased graphite nitrogen (from 10.6 to 30.1%) gradually by the precise design of the nitrogen structure in carbon sources (e.g., primary amines, secondary amines, tertiary amines, and liking graphite-nitrogen), the energy gap of CDs reduced, resulting in adjustable wavelength from visible to near-infrared range (from 461 nm/527 nm to 650 nm/676 nm). Furthermore, N-CDs exhibited a selective affinity for nucleic acids, especially RNA. Therefore, N-CDs support an efficient platform for real-time tracking of RNA dynamic changes in cells.

Julolidine-based small molecular probes for fluorescence imaging of RNA in live cells

Intracellular RNA imaging with organic small molecular probes has been an intense topic, although the number of such reported dyes, particularly dyes with high quantum yields and long wavelength excitation/emission, is quite limited. The present work reports the design and synthesis of three cationic julolidine-azolium conjugates (OX-JLD, BTZ-JLD and SEZ-JLD) as turn-on fluorescent probes with appreciably high quantum yields and brightness upon interaction with RNA. A structure-efficiency relationship has been established for their potential for the interaction and imaging of intracellular RNA. Given their chemical structure, the free rotation between the donor and the acceptor gets restricted when the probes bind with RNA resulting in strong fluorescence emission towards a higher wavelength upon photoexcitation. A detailed investigation revealed that the photophysical properties and the optical responses of two probes, viz. BTZ-JLD and SEZ-JLD, towards RNA are very promising and qualify them to be suitable candidates for biological studies, particularly for cellular imaging applications. The probes allow imaging of intracellular RNA with prominent staining of nucleoli in live cells under a range of physiological conditions. The results of the cellular digest test established the appreciable RNA selectivity of BTZ-JLD and SEZ-JLD inside the cellular environment. Moreover, a comparison between the relative intensity profile of SEZ-JLD before and after the RNA-digestion test inside the cellular environment indicated that the interference of cellular viscosity in fluorescence enhancement is insignificant, and hence, SEZ-JLD can be used as a cell membrane permeable cationic molecular probe for deep-red imaging of intracellular RNA with a good degree of selectivity.

Zero-Dimensional Carbon Nanomaterials for Fluorescent Sensing and Imaging

Advances in nanotechnology and nanomaterials have attracted considerable interest and play key roles in scientific innovations in diverse fields. In particular, increased attention has been focused on carbon-based nanomaterials exhibiting diverse extended structures and unique properties. Among these materials, zero-dimensional structures, including fullerenes, carbon nano-onions, carbon nanodiamonds, and carbon dots, possess excellent bioaffinities and superior fluorescence properties that make these structures suitable for application to environmental and biological sensing, imaging, and therapeutics. This review provides a systematic overview of the classification and structural properties, design principles and preparation methods, and optical properties and sensing applications of zero-dimensional carbon nanomaterials. Recent interesting breakthroughs in the sensitive and selective sensing and imaging of heavy metal pollutants, hazardous substances, and bioactive molecules as well as applications in information encryption, super-resolution and photoacoustic imaging, and phototherapy and nanomedicine delivery are the main focus of this review. Finally, future challenges and prospects of these materials are highlighted and envisaged. This review presents a comprehensive basis and directions for designing, developing, and applying fascinating fluorescent sensors fabricated based on zero-dimensional carbon nanomaterials for specific requirements in numerous research fields.

Dual-Emission Carbon-Dot Ratiometric Fluorescence Sensor for Morphine Recognition in Biological Samples

Herein, a novel nitr[ogen-doped carbon dot (N-CD) fluorescence sensor with a dual emission ratio is developed using the microwave-assisted synthesis of m-phenylenediamine and spermidine. As a result of the fluorescence inner filtration effect (IFE) effect between morphine (MOR) and N-CD, the blue fluorescence of N-CDs at 350 nm was reduced in the presence of MOR, whereas the fluorescence of N-CDs at 456 nm increased substantially. The results demonstrated that the approach has a tremendous potential and that the linear range of MOR detection is 0.25-25 µg/mL, with a 71.8 ng/mL detection limit. Under UV light, the blue fluorescent system is easily visible to the naked eye. More significantly, the sensor proved successful in providing satisfactory results for the speciation measurement of MOR in a variety of biological samples.

See the Unseen: Red-Emissive Carbon Dots for Visualizing the Nucleolar Structures in Two Model Animals and In Vivo Drug Toxicity

Nucleolus, which participates in many crucial cellular activities, is an ideal target for evaluating the state of a cell or an organism. Here, bright red-emissive carbon dots (termed CPCDs) with excitation-independent/polarity-dependent fluorescence emission are synthesized by a one-step hydrothermal reaction between congo red and p-phenylenediamine. The CPCDs can achieve wash-free, real-time, long-term, and high-quality nucleolus imaging in live cells, as well as in vivo imaging of two common model animals-zebrafish and Caenorhabditis elegans (C. elegans). Strikingly, CPCDs realize the nucleolus imaging of organs/flowing blood cells in zebrafish at a cellular level for the first time, and the superb nucleolus imaging of C. elegans suggests that the germ cells in the spermatheca probably have no intact nuclei. These previously unachieved imaging results of the cells/tissues/organs may guide the zebrafish-related studies and benefit the research of C. elegans development. More importantly, a novel strategy based on CPCDs for in vivo toxicity evaluation of materials/drugs (e.g., Ag⁺ ), which can visualize the otherwise unseen injuries in zebrafish, is developed. In conclusion, the CPCDs represent a robust tool for visualizing the structures and dynamic behaviors of live zebrafish and C. elegans, and may find important applications in cell biology and toxicology.

An insight into embryogenesis interruption by carbon nitride dots: can they be nucleobase analogs?

The carbon nitride dot (CND) is an emerging carbon-based nanomaterial. It possesses rich surface functional moieties and a carbon nitride core. Spectroscopic data have demonstrated the analogy between CNDs and cytosine/uracil. Recently, it was found that CNDs could interrupt the normal embryogenesis of zebrafish. Modifying CNDs with various nucleobases, especially cytosine, further decreased embryo viability and increased deformities. Physicochemical property characterization demonstrated that adenine- and cytosine-incorporated CNDs are similar but different from guanine-, thymine- and uracil-incorporated CNDs in many properties, morphology, and structure. To investigate the embryogenesis interruption at the cellular level, bare and different nucleobase-incorporated CNDs were applied to normal and cancerous cell lines. A dose-dependent decline was observed in the viability of normal and cancerous cells incubated with cytosine-incorporated CNDs, which matched results from the zebrafish embryogenesis experiment. In addition, nucleobase-incorporated CNDs were observed to enter cell nuclei, demonstrating a possibility of CND-DNA interactions. CNDs modified by complementary nucleobases could bind each other via hydrogen bonds, which suggests nucleobase-incorporated CNDs can potentially bind the complementary nucleobases in a DNA double helix. Nonetheless, neither bare nor nucleobase-incorporated CNDs were observed to intervene in the amplification of the zebrafish polymerase-alpha 1 gene in quantitative polymerase chain reactions. Thus, in conclusion, the embryogenesis interruption by bare and nucleobase-incorporated CNDs might not be a consequence of CND-DNA interactions during DNA replication. Instead, CND-Ca²⁺ interactions offer a plausible mechanism that hindered cell proliferation and zebrafish embryogenesis originating from disturbed Ca²⁺ homeostasis by CNDs. Eventually, the hypothesis that raw or nucleobase-incorporated CNDs can be nucleobase analogs proved to be invalid.

Fluorescent Carbon Dot-Supported Imaging-Based Biomedicine: A Comprehensive Review

Carbon dots (CDs) provide distinctive advantages of strong fluorescence, good photostability, high water solubility, and outstanding biocompatibility, and thus are widely exploited as potential imaging agents for in vitro and in vivo bioimaging. Imaging is absolutely necessary when discovering the structure and function of cells, detecting biomarkers in diagnosis, tracking the progress of ongoing disease, treating various tumors, and monitoring therapeutic efficacy, making it an important approach in modern biomedicine. Numerous investigations of CDs have been intensively studied for utilization in bioimaging-supported medical sciences. However, there is still no article highlighting the potential importance of CD-based bioimaging to support various biomedical applications. Herein, we summarize the development of CDs as fluorescence (FL) nanoprobes with different FL colors for potential bioimaging-based applications in living cells, tissue, and organisms, including the bioimaging of various cell types and targets, bioimaging-supported sensing of metal ions and biomolecules, and FL imaging-guided tumor therapy. Current CD-based microscopic techniques and their advantages are also highlighted. This review discusses the significance of advanced CD-supported imaging-based in vitro and in vivo investigations, suggests the potential of CD-based imaging for biomedicine, and encourages the effective selection and development of superior probes and platforms for further biomedical applications.

Design and Synthesis of Nanosensor Based on Unsaturated Double Bond Functional Carbon Dots for Phenylephrine Detection Using Bromine As a Bridge

In recent years, carbon dots (CDs) have attracted great research interest in the field of nanochemosensors due to their fascinating optical properties. However, synthesis of CDs with novel recognition groups in a convenient method is still an area to be explored urgently. In this work, we reported a simple strategy to prepare fluorescent CDs with carbon-carbon double bonds (C═C) as the characteristic structure for phenylephrine (PHE) identification and detection. The itaconic acid and polyethylenimine (PEI) were selected as precursors to fabricate highly emissive CDs under the hydrothermal cross-linking and carbonization process. The fluorescence of designed CDs at 465 nm can be effectively quenched by bromine aqueous solution due to the electrophilic addition reaction with the double bonds. On the other hand, the presence of PHE can inhibit fluorescence quenching by bromine-consumption of a substitution reaction. Inspired by the novel findings, a convenient assay for PHE determination was established using the fluorescence of C═C bond functional CDs as an output signal and bromine as a bridge. The method demonstrated here provided a unique way to develop CD-based nanosensors.

A "Polymer Template" Strategy for Carbonized Polymer Dots with Controllable Properties

Limited avenues are available for property control of carbonized polymer dots (PDs) owing to the unsatisfactory understanding of PDs" formation. Herein, a de novo "polymer template" strategy is presented for PDs with customizable functional surface groups (FSG), size, and underlying fluorescence, with a detailed mechanism. The strategy relies on novel di-active site polymers (DASPs) prepared from alkenyl azides via [3+2] cycloaddition and guanidino hydrolysis. Benefiting from these specific reactions, the DASPs were convenient for mass production and stable for storage, and could be transformed to PDs upon addition of nucleophilic agents through nucleophilic addition and substitution at 70 °C. By regulating the types of alkenyl azides, nucleophilic agents, and reaction conditions, the as-prepare PDs could be tailored with controlled types of core, FSG, and particle size, as well as fluorescence properties of quantum yield from 8.2-55.6 %, and emission maximum from 380-500 nm. These specialties make this "polymer template" strategy a promising start for building PDs-based sensor platforms. Moreover, the strategy could further our understanding towards PDs' formation, and open up a new way to customize PDs for specific needs in the fields of analysis, catalysis, images, etc.

Facile synthesis of ultrahigh fluorescence N,S-self-doped carbon nanodots and their multiple applications for H2S sensing, bioimaging in live cells and zebrafish, and anti-counterfeiting

Green-emissive N,S-self-doped carbon nanodots (N,S-self-CNDs) with an ultrahigh fluorescence (FL) quantum yield (QY) of 60% were synthesized using methyl blue as the only source by a facile hydrothermal approach. The -NH- and -SOx- groups of methyl blue were simultaneously used as nitrogen and sulfur co-dopants to dope into CNDs. The prepared N,S-self-CNDs have an extremely large Stokes shift (∼130 nm) and excitation-independent fluorescence, and are demonstrated to have multiple applications for H2S sensing, bioimaging and anti-counterfeiting. Taking advantage of their excellent optical properties, N,S-self-CNDs could act as a label-free nanoprobe for the detection of H2S. The FL of N,S-self-CNDs could be significantly quenched by H2S because of dynamic quenching, along with excellent selectivity toward H2S from 0.5-15 μM with a detection limit of 46.8 nM. They were successfully employed for the analysis of H2S content in actual samples. Additionally, the nanoprobe was extended to bioimaging in both living PC12 cells and zebrafish, and monitoring H2S in live cells. Furthermore, N,S-self-CNDs have been used to prepare highly fluorescent polymer films by incorporating N,S-self-CNDs in polyvinyl alcohol (PVA). The as-prepared N,S-self-CNDs/PVA films show a prominent dual-mode FL property, implying that they are potential nanomaterials in the anti-counterfeiting field.

Fluorescent probe based on N-doped carbon dots for the detection of intracellular pH and glutathione

Carbon dots (CDs) as fluorescent probes have been widely exploited to detect biomarkers, however, tedious surface modification of CDs is generally required to achieve a relatively good detection ability. Here, we synthesized N-doped carbon dots (N-CDs) from triethylenetetramine (TETA) and m-phenylenediamine (m-PD) using a one-step hydrothermal method. When the pH increases from 3 to 11, the fluorescence intensity of the N-CDs gradually decreases. Furthermore, it displays a linear response to the physiological pH range of 5-8. Au³⁺ is reduced by amino groups on the surface of N-CDs to generate gold nanoparticles (AuNPs), causing fluorescence quenching of the N-CDs. If glutathione (GSH) is then added, the fluorescence of the N-CDs is recovered. The fluorescence intensity of the N-CDs is linearly correlated with the GSH concentration in the range of 50-400 μM with a limit of detection (LOD) of 7.83 μM. The fluorescence probe was used to distinguish cancer cells from normal cells using pH and to evaluate intracellular GSH. This work expands the application of CDs in multicomponent detection and provides a facile fluorescent probe for the detection of intracellular pH and GSH.

RNA-responsive fluorescent carbon dots for fast and wash-free nucleolus imaging

RNA as a carrier of genetic information plays a critical role in various physiological processes. RNA-rich nucleolus is usually employed as an important biomarker for many malignant diseases. Herein, RNA-responsive fluorescent carbon dots (CDs) were synthesized by a simple microwave method. Due to the presence of cationic benzothiazolium groups in the CDs, a "turn-on" fluorescence signal was achieved between CDs and RNA. The CDs exhibit excellent RNA selectivity and a good linear relationship with a detection limit of 0.62 μg/mL. The small particle size, polarity sensitivity and RNA response behavior of CDs realized fast and wash-free nucleolus imaging effectively. Overall, these CDs provide a powerful potential tool for monitoring cell nucleus activity and elucidating RNA dynamics.

Synthesis of Carbon Dots by Varying Doped Elements and Application in Serine Detection

Carbon dots (CDs) with different doping elements were successfully synthesized via a simple hydrothermal strategy. 3-amino-4-chlorophenylboronic acid, 3-aminobenzeneboronic acid, aniline, and benzene were used as precursors, respectively. The B/N co-doping CDs (BNCDs) derived from 3-aminobenzeneboronic acid show brightest fluorescence among the CDs products with quantum yield at 0.15. The fluorescence of BNCDs exhibits good photostability and excitation-independent emission behavior. The bright blue emission of BNCDs can be quenched by serine, which is a kind of neutral aliphatic amino acid containing hyroxyl groups with polarity. It is possibly due to the molecular collision between excited state of BNCDs and the ground state of serine. BNCDs can be served as fluorophore probe for the assay of serine based on the efficient quenching effect. The approach for the determination of serine shows a high sensitivity with a detection limit at 0.14 nM, which is lower than those of previous works. Furthermore, the present BNCDs system can be employed to monitor serine in real food and biological samples. The strategy may be a potential way for the application in food safety and biomedicine fields.

Fluorescent Labeling of Proteins of Interest in Live Cells: Beyond Fluorescent Proteins

Live cell imaging brings us into a new era of direct visualization of biological processes and molecular dynamics in real time. To visualize dynamic cellular processes and virus-host interactions, fluorescent labeling of proteins of interest is often necessary. Fluorescent proteins are widely used for protein imaging, but they have some intrinsic deficiencies such as big size, photobleaching, and spectrum restriction. Thus, a variety of labeling strategies have been established and continuously developed. To protect the natural biological function(s) of the protein of interest, especially in viral life cycle, in vivo labeling requires smaller-sized tags, more specificity, and lower cytotoxicity. Here, we briefly summarized the principles, development, and their applications mainly in the virology field of three strategies for fluorescent labeling of proteins of interest including self-labeling enzyme derivatives, stainable peptide tags, and non-canonical amino acid incorporation. These labeling techniques greatly expand the fluorescent labeling toolbox and provide new opportunities for imaging biological processes.

Highly sensitive fluorescent carbon dots probe with ratiometric emission for the determination of ClO−

A ratiometric fluorescent N,S co-doped carbon dots (N,S-CD) probe for ClO⁻ has been facilely obtained via a one-step hydrothermal method.

Carbon dot-based composites for catalytic applications

We summarize the construction methods and influencing factors of CDs-based composites and discuss their catalytic applications, including photocatalysis, chemical catalysis, peroxidase-like catalysis, Fenton-like catalysis and electrocatalysis.

Synthesis, applications and potential photoluminescence mechanism of spectrally tunable carbon dots

Due to the prominent characteristics of carbon-based luminescent nanostructures (known colloquially as carbon dots), such as inexpensive precursors, excellent hydrophilicity, low toxicity, and intrinsic fluorescence, these nanomaterials are regarded as potential candidates to replace traditional quantum dots in some applications. As such, research in the field of carbon dots has been increasing in recent years. In this mini-review, we summarize recent progress in studies of multicolor carbon dots focusing on potential photoluminescence (PL) mechanisms, strategies for effective syntheses, and applications in ion/molecule and temperature sensing, light emitting diodes and high-resolution bioimaging techniques.

Carbon Dots for In Vivo Bioimaging and Theranostics

Carbon dots (CDs), a kind of carbon material discovered accidentally, exhibit unexpected advantages in fluorescence imaging/sensing such as photostability, biocompatibility, and low toxicity. For emerging theranostics, an interdiscipline created by integrating therapy and diagnostics, CDs are good candidates when they are combined with targeted chemo/gene/photodynamic/photothermal therapeutic moieties. Here, the development of CDs in nanomedicine is reviewed from their use as original imaging agents and/or drug carriers to multifunctional theranostic systems. Finally, the challenges and prospects of the next-generation of CD-based theranostics for clinical applications are also discussed.

Carbon dots: Applications in bioimaging and theranostics

Carbon dots are a carbonaceous nanomaterial that were discovered accidentally and are now drawing significant attention as a new quantum-sized fluorescent nanoparticle. Carbon dots are biocompatible, non-toxic, photostable, and easily functionalized with good photoluminescence and water solubility. Due to these unique properties, they are used broadly in live cell imaging, catalysis, electronics, biosensing, power, targeted drug delivery, and other biomedical applications. Here, we review the recent development of carbon dots in nanomedicine from their use in drug carriers to imaging agents to multifunctional theranostic systems. Finally, we discuss the challenges and views on next-generation carbon dot-based theranostics for clinical applications.

Observation of Acetylcholinesterase in Stress-Induced Depression Phenotypes by Two-Photon Fluorescence Imaging in the Mouse Brain

Oxidative stress in depression is a prime cause of neurotransmitter metabolism dysfunction in the brain. Acetylcholinesterase (AChE), a key hydrolase in the cholinergic system, directly determines the degradation of neurotransmitters. However, due to the complexity of the brain and lack of appropriate in situ imaging tools, the mechanism underlying the changes in AChE activity in depression remains unclear. Hence, we generated a two-photon fluorescence probe (MCYN) for real-time visualization of AChE with excellent sensitivity and selectivity. AChE can specifically recognize and cleave the carbamic acid ester bond in MCYN, and MCYN emits bright fluorescence at 560 nm by two-photon excitation at 800 nm. By utilizing MCYN to monitor AChE, we discovered a significant increase in AChE activity in the brains of mice with depression phenotypes. Notably, with the assistance of a two-photon fluorescence imaging probe of the superoxide anion radical (O₂^•-), in vivo visualization for the first time revealed the positive correlation between AChE and O₂^•- levels associated with depressive behaviors. This finding suggests that oxidative stress may induce AChE overactivation, leading to depression-related behaviors. This work provides a new and rewarding perspective to elucidate the role of oxidative stress regulating AChE in the pathology of depression.